1. Field of the Invention
The present invention is directed to a system for reading Fourier holograms, in particular to a method of reading a Fourier hologram recorded on a holographic storage medium and to a holographic storage system.
Holographic data storage is based on the concept of recording the interference pattern of a data-encoded signal beam (also referred to as an object beam) carrying the data and of a reference beam at a holographic storage medium. Generally a spatial light modulator (SLM) is used for creating the object beam and the holographic storage medium can be for example a photopolymer or photorefractive crystal or any other material which is suitable for recording the relative amplitudes of, and phase differences between the object beam and the reference beam. After a hologram is created in the storage medium, projecting the reference beam into the storage medium interacts and reconstructs the original data-encoded object beam, which can be detected by a detector such as a CCD-array camera or the like. The reconstructed data-encoded object beam is generally referred to in the art as the reconstructed hologram itself. According to this terminology reconstruction of a hologram means the reconstruction of the original data-encoded object beam; and reading of the hologram means detecting the reconstructed hologram, in particular an image of the reconstructed hologram. This terminology is adapted in the present specification.
2. Description of Related Art
The writing of holograms is greatly influenced by the spatial overlap of the object beam and the reference beam, while hologram reading is strongly affected by the relative position of the reconstructing reference beam and the hologram stored in the storage medium. Reading of a holographic storage medium can be relatively easily achieved if both the reference beam and the object beam cover a relatively large spot on the surface of the storage medium. The tolerance of displacement between the centre of the hologram and the centre of the reference beam is approximately 10% of the size of the beam diameter, which is usually within the mechanical limits of conventional systems. However, decreasing the hologram size leads to a higher demand on alignment of the reference beam and the hologram when reading the medium. High-precision alignment can also be necessary for example, in case of multiplexing and/or security encrypting the stored holographic data.
There are many known methods of multiplexing and/or encrypting holograms. Such methods may involve phase coding the object beam and/or the reference beam both in the real and/or in the Fourier-plane. A method of, and device for, phase coded multiplexing and encrypting by phase coding the reference beam is disclosed in WO 02/05270 A1. When applying phase coded multiplexing or encrypting the tolerance of displacement between the centre of the reference beam and the hologram during reconstruction of the hologram can drop to 1% of the beam diameter. Misalignment of the beam and the hologram is generally associated with the misalignment of the optical components of the system, which can be due to mechanical shocks, temperature changes, etc. It is also a common problem of systems designed to receive removable storage medium, such as holographic identification cards.
U.S. Pat. No. 7,116,626 B1 teaches a micro-positioning method to overcome the above identified problem of misalignment. The object of the described method is to increase the performance of a holographic storage system, i.e., the quality of the modulated image, by ensuring the correct alignment of various components of the system such as an SLM with various devices, such as light sources, lenses, detectors, and the storage medium. The alignment technique focuses on “pixel matching” that is aligning the pixels of an SLM, the stored holographic image and the detector so that each pixel of the SLM is projected onto a single pixel of the detector resulting in better data recovering efficiency. The method involves physically moving all or some of said components and a servomechanism is suggested to control the positioning of the components based on feedback associated with a misalignment derived from the detected image.
Various exemplary methods are described for determining the misalignment of the detected images. In one example, the misalignment is based on the measurement of a channel metric associated with the detected image. The channel metric is generally a scalar quantity indicative of the pixel misalignment, e.g. average pixel intensity or SNR. Channel metrics indicate the magnitude or degree of misalignment but not the direction of the misalignment. Consequently, in order to minimise pixel registration errors at least one component of the system needs to be moved and the channel metric has to be recalculated at the new component position. It may take a large number of steps to find the optimal position of the components with respect to each other, which can be very time consuming.
In another example, the misalignment is based on the measurement of a page metric associated with the detected image. Page metrics generally include reference pixels, i.e. known pixel patterns or registration marks, such as blocks of pixels located within the user encoded data or in border regions around the user encoded data. The known pixel patterns may be detected and used to determine misalignments of various components of the holographic storage system. The drawback of this solution is that the holograms have to be provided with the reference pixel blocks, which occupy precious data space if too large and are difficult to locate in the image if too small. The reference pixel blocks serve for calculating the point spread function (PSF) of the system, which is a key information used by known servo methods responsible for aligning the components of the system. However it would be desirable to have servo methods which do not require the PSF of the system.